Fuel cells are able to convert hydrogen fuel to electric power in a simple, efficient process with low pollutant emissions, and are therefore of considerable commercial interest. A number of fuel cell designs have been introduced, using various operating temperatures. Both low temperature fuel cells, such as proton exchange membrane fuel cells, and high temperature fuel cells, such as solid oxide fuel cells, require fuel feeds containing low concentrations of sulfur.
To be practical for most commercial applications, fuel cell systems must be compact, efficient, and able to use widely available fuels. Hydrogen gas is not a widely distributed fuel, and is difficult to store and transport. Alcohol fuels, such as methanol and ethanol, can be readily reformed to hydrogen gas, but are also not widely available. Petroleum distillate fuels such as gasoline, diesel, Jet A, JP-8, and kerosene, are widely distributed and readily available, have higher specific heating values than alcohol fuels, and can be reformed to hydrogen gas for use in fuel cells using a variety of methods, including partial oxidation, steam reforming, and autothermal reforming. Of all-available reforming methods, steam reforming of distillate fuels is preferred, because it provides the greatest hydrogen yield with no nitrogen dilution, and because steam reforming does not require use of an air compressor, thus allowing for greater overall reforming efficiency and more compact system design.
Conversion of a liquid distillate fuel to hydrogen by steam reforming involves vaporization of the fuel feed and mixing of the fuel vapor with steam, followed by passage of the steam/fuel mixture over a catalytic surface, on which hydrocarbon molecules in the fuel feed undergo a series of reactions with steam molecules, resulting in hydrogen and carbon dioxide products, with lesser amounts of methane and carbon monoxide.
While distillate fuels are more widely distributed than alcohol fuels or natural gas, they also present some unique complications in steam reforming processes. Specifically, coke formation and sulfur poisoning are two common problems in steam reforming of distillate fuels.
Coking of the reforming catalyst surface may result from imperfect mixing of fuel vapor and steam at the reforming reactor inlet. Imperfect mixing leads to gradients in temperature and steam/carbon ratio over the catalyst surface, with coke formation occurring in those regions where temperature or steam/carbon ratios are low.
Steam reforming of aromatic hydrocarbon species is generally more likely to lead to coke formation than reforming of aliphatic hydrocarbons, and heavier hydrocarbons are more likely to form coke than lighter hydrocarbons. Coke formation initially causes lowered catalyst activity, and ultimately can lead to plugging of the reforming reactor. Increasing the steam/carbon ratio of the steam/fuel mixture can diminish coking; however, this results in wasted steam and lower overall process efficiency.
Coke accumulation can also occur during vaporization of the distillate fuel feed. Heavier distillate fuels such as JP-8, kerosene, and diesel normally contain a significant amount of nonvolatile hydrocarbons, which may accumulate and gradually decompose to coke in the vaporization chamber of a reforming apparatus. JP-8, for example, may contain up to 1.5 volume percent nonvolatile residue, which could rapidly accumulate in a vaporization chamber. Coke accumulation due to decomposition of nonvolatile hydrocarbon species in the vaporization chamber of a reforming apparatus could be diminished or eliminated if the apparatus were designed to provide for continuous separation and removal of nonvolatiles from the process stream.
All ordinary distillate fuels contain some amount of sulfur, which may be present as a range of organosulfur compounds, including alkyl sulfides and disulfides, alkyl mercaptans, and substituted or unsubstituted thiophenes, benzothiophenes, and dibenzothiophenes. All of these sulfur species are capable of poisoning reformer and fuel cell system components. Sulfur species present in the fuel feed can be removed upstream of the steam reforming reactor by adsorption. However, the adsorption capacities of conventional adsorbents such as zinc oxide vary significantly from one organosulfur species to another. Hydrogen sulfide is readily adsorbed, while alkyl sulfides are more difficult to remove, thiophenes more difficult still, and substituted benzothiophenes are highly stable and only weakly adsorbed. Direct desulfurization of the liquid distillate fuel feed to acceptable sulfur concentrations using conventional adsorbents may therefore require impractically large adsorbent beds. More effective sulfur removal could be achieved if all of the sulfur in the distillate fuel feed could be converted to hydrogen sulfide prior to desulfurization using adsorbent beds.
All distillate fuels contain both aliphatic and aromatic hydrocarbon compounds. Typically, the aromatic content of a distillate fuel comprises between 15 and 40 weight percent. Steam reforming of distillate fuels necessarily involves several endothermic processes, including steam generation from liquid water, vaporization of the liquid fuel feed, and the steam reforming chemical reactions. In conventional steam reforming systems, the heat required to drive these endothermic processes is supplied by combusting some portion of the fuel feed. The thermodynamics of the reforming process require that at least 30% of the fuel feed be combusted to balance the heat requirements of steam reforming the other 70%. However, indiscriminate combustion of the fuel feed is thermally inefficient, because hydrogen-rich aliphatic species in the feed are combusted along with hydrogen-poor aromatic species. Aliphatic hydrocarbon compounds have higher hydrogen/carbon ratios than aromatics, so that steam reforming of aliphatics produces more hydrogen than steam reforming of aromatics. Conversely, aromatic hydrocarbon compounds require less air for stoichiometric combustion, and so are more advantageously used as combustion fuels. Greater overall thermal efficiency of the reforming process could be achieved if the aromatic content of the feed could be preferentially combusted, while the aliphatic content of the feed is preferentially steam reformed.
Processes and apparatus for steam reforming of hydrocarbon fuels are well known. The prior art discloses a considerable variety of reforming process designs, some of which use fractionator or catalytic cracking reactor components to pretreat the hydrocarbon fuel feed. For example, U.S. Publication No. U.S. 2002/0031690 to Shimazu et al., discloses a fuel reforming apparatus, which may include an autothermal cracking unit, in which the cracking process is accomplished using partial oxidation to provide the thermal energy needed for thermally cracking the fuel. Gas/liquid separation is not used to separate light hydrocarbons from uncracked residue downstream from the cracking reactor, making the Shimazu process unsuitable for use with middle distillate fuels such as diesel, kerosene or JP-8 which contain significant amounts of higher molecular weight aromatic species. U.S. Pat. No. 6,254,839 to Clawson et al., discloses an apparatus for converting hydrocarbon fuel into hydrogen gas and carbon dioxide. Because the heavy fractions are difficult to reform, this apparatus may include a fractionator component, which separates the hydrocarbon fuel feed into heavy and light fractions. The heavy fractions are subsequently sent to a partial oxidation zone while the light fractions are heated in an adjacent zone. The two fractions are then remixed in a mixing zone, and the entire mixture directed to a reforming zone. In both of these prior art examples, all sulfur present in the hydrocarbon fuel feed must be removed by adsorption, and the oxidation and reforming processes are not segregated, so that a pressurized oxygen source is required, and any nitrogen introduced along with the oxygen dilutes the final product gas.